🍒 applet: Fix crash when opening context menu with no contents
Do not try to open the context menu via the right mouse button or the Menu key press when it has no contents, because that leads to a crash.
The menu button was already guarded against this because it gets hidden in that case.
BUG: 470847 FIXED-IN: 5.27.6
(cherry picked from commit 65936f7a)